It is known that certain industrial hide treatments, such as tanning, retanning and greasing, are performed in the wet, i.e. by immersing the hides in more or less dense water, oil or grease-based liquids, which contain chemical substances of an organic or inorganic nature and have the function of inhibiting bacterial or enzymatic processes or protecting the hides and giving them a specific aesthetic appearance.
After the abovementioned treatments, the wetted hides must be dried and extended before proceeding with the subsequent processing steps.
It is also known that the surface area of the hides is a factor of vital importance for the cost-efficiency of a tannery, in that the price of the finished products—the final quality being equal—is determined on the basis of on this parameter. Therefore, one of the main objectives of tanneries is that of producing hides having the largest surface area possible, in a manner which is compatible with the other qualitative demands.
In the past, in order to perform the stretching operation, traditional straight-edge blades, which require a large expenditure of energy on the part of the operators, with limited effectiveness, were used.
This manual operation has gradually been replaced by other automatic operations, generally called “setting out”, which are performed, for example, using wringing machines having spiralled cylinders arranged opposite each other. Alternatively, the hides may be pressed and stretched by means of presses provided with pairs of felt belts which act on the hides during the feeding thereof.
Another method of extending the hides consists in so-called “pasting”, which envisages a paste being deposited on the coarse side of the wetted hides, said paste having the function of making said hides stick to glass, porcelain or metal plates. The hides treated in this way pass into a tunnel drier until they lose a significant proportion of their moisture. Once dried, the hides are separated and are in the form of flat sheets, ready for finishing and subsequent processing.
Another method of stretching the hides, known in technical jargon as “toggling”, envisages the use of grippers or clamps applied along the edges of the hides and directed radially towards the outside. The clamps are secured to frames which keep the hides extended while they are passed through a tunnel drier or along a path in a controlled atmosphere. Optionally, the frames are provided with plates for supporting the hides in sections which can be separated, as described, for example, in U.S. Pat. No. 2,834,147. A significant drawback of said known method lies in the fact that the areas affected by the grippers are put under greater stress than the central areas, in such a way that they are irreversibly damaged, giving rise to so-called “pockets” or blisters which are due to separation and creeping of the fibres with respect to the dermis.
Moreover, the thickness of the hides is less in the gripped areas than the central areas and therefore varied stretching and thinning of the hides occurs, with consequent lower quality of the finished product.
Another known method, described in French application 2,602,795, envisages the use of a pair of flat plates on which respective elastically deformable mats, generally made of rubber, are placed. By compressing the hides between the mats arranged opposite each other and applying a longitudinal elongation thereto, a stretching of the hides through friction is achieved. Moreover, hot air, which has the function of drying the hides, is directed between the mats. Unfortunately, the heat used for heating the hides also damages the rubber mats and therefore limits their working life. Moreover, as the working temperature cannot exceed certain maximum values, the drying time increases, reducing the efficiency of the process.
Said known stretching methods have the drawback of subjecting the hides to a mechanical stretching action which is concentrated in particular along the peripheral edge thereof. When acting on said zones, which are the thinnest areas, it is necessary to restrict the magnitude of the stresses in order to avoid breakage. This restricts the increase in the surface area and therefore the effectiveness of the method.
Moreover, the stresses are not uniformly distributed and, above all, are not applied towards the centre of the hide, which has the greatest thickness and could therefore allow the greatest stretching.
British patent application 2,236,111 describes a method for increasing the total surface area of chrome-tanned or “wet blue” tanned hides, which envisages the moist hides being placed on a flat surface and being subjected to a simultaneous action of longitudinal traction, by means of grippers, and compression by means of rollers displaced longitudinally with an alternating motion which is repeated following rotation of the hides through about 90° C. This treatment allows an increase in surface area of about 10%.
However, even this solution, like the preceding solutions, does not eliminate the problem of mechanically stressing the peripheral zones of the hides and not achieving a uniform stretching thereof.
Moreover, the application of this method does not allow stretching of the fibres towards the central part thereof and therefore the liquids absorbed in this zone are retained for longer than in other zones, increasing the drying times.